Tracked vehicle drive system

ABSTRACT

A drive system includes a vehicle propulsion device having a speed change gear box which drives tracks through a pair of planetary gears and a steering drive having a pair of variable hydrostatic drives. The hydrostatic drives engage a neutral shaft which superimposes rotation on the planetary gears to provide steering control for the tracks. The neutral shaft is separable into two segments. A differential reversing gear may be employed to selectively couple the neutral shaft segments. When the vehicle travels at high speed, the neutral shaft segments are coupled for unitary rotation. At low vehicle speeds, the propulsion drive is blocked and the neutral shaft segments are uncoupled so that both propulsion and steering are provided through the hydrostatic drives. For combined steering control and regulation of slow travel speeds as a function of the requirements of specific operating equipment carried by the vehicle, a control apparatus operates as a function of electrical signals for regulating travel speed and provides, through a linkage, adjustment of the hydrostatic unit controls and, in addition, superimposes upon the adjustment, modifications required for steering the vehicle.

BACKROUND OF THE INVENTION

1. Field of the invention

The present invention relates generally to power transmission in trackedvehicles such as tanks and more specifically to a drive system wherein ahydrostatic steering drive is employable to provide vehicle propulsionand steering control at low speeds.

2. Backround art

Drive assemblies for tracked vehicles having a mechanical speed changegear box employed in a propulsion drive with an infinitely variablesuperimposed steering drive have been disclosed in U.S. Pat. No.4,428,445, U.S. Pat. No. 4,327,603 and U.S. Pat. No. 4,280,579. Inaddition, tracked vehicle drive systems including steering drives havinghydrostatic or hydrodynamic units for driving a neutral shaft weredisclosed in U.S. Pat. No. 3,371,734 and the following patent documentsof the Federal Republic of Germany; DEP 14 80 725, DEP 860,601; DAS 1929 380; DOS 23 22 457 and DAS 11 74 182.

The vehicle drive systems of the past were incapable of providing bothhigh speed vehicle travel, in the order of, for example, 60 km/hr withprecise steering control at such high speeds while maintaining theability to accurately drive the vehicle at relatively slow speeds, forexample, in the order of 0.2 to 5.0 km/hr as may be neccesitated byspecial circumstances. In addition, prior drive systems lacked theability to provide controlled driving in a straight direction withoutmanual correction for ground conditions, for example, inclination orslope of the travel path or different driving resistances due to groundterrain and/or drive system component variations.

A further deficiency encoutered with prior drive systems related tocontrol apparatus for steering and/or propulsion drives. In full trackvehicles such as tanks, high demands were made of the power transmissionsystems including propulsion and steering drives as well as controlapparatus for such drives. Such demands dealt primarily with efficiencyand operation safety for both military reasons and accident avoidance.For safety purposes, it was desired to operate control elements throughmechanical linkages yet control systems heretofore lacked the ability toprovide mechanical steering linkage controls which could beautomatically varied in position as a function of predeterminedadditional parameters.

SUMMARY OF THE INVENTION

A tracked vehicle drive system includes a propulsion drive and asteering drive. While the propulsion drive functions only to propel thevehicle by driving a pair of planetary gears which engage the tracks orchains, the steering drive functions to provide, at slow speeds, bothsteering and vehicle propulsion.

The steering drive includes a pair of hydrostatic drive units whichengage a neutral shaft. The neutral shaft engages the planetary gears.With the vehicle drive system in a "travel" mode, e.g. when traveling athigh speeds, the neutral shaft superimposes a steering control input onthe planetary gears which are primarily driven through the propulsiondrive.

At low speeds, with the drive system in a "work" mode, the neutral shaftis separated into segments each drivable by one of the hydrostatic unitsand the propulsion drive input to the planetary gears is blocked. Adifferential reversing gear having two couplings may be used forcoupling the neutral shaft segments.

With both couplings open, the shaft segments are separated for slowspeed vehicle propulsion and steering (work mode). When one coupling isclosed with the other opened, the neutral shaft is unitarily coupled forsuperimposed steering control when the vehicle is propelled through thepropulsion drive (travel mode). With one coupling open and the otherclosed, the reversing gear couples the segments for slow speedstabilized straight travel ("special work" mode) wherein the drivingtorque of the hydrostatic units are divided proportional to thedifferent driving resistances at the tracks to provide the same speedfor each track.

A control apparatus is provided for adjusting the position of thehydrostatic unit controls as a function of predetermined vehicle speedrequirements mandated by equipment such as a mine detector. The controlapparatus includes a rotatable driver mechanically linked to an operatorsteering control, and a pair of coaxial rotatable driven elements eachmechanically linked to a control element of a steering drive. Aconcentric tube interconnects the driver and driven elements. The tubeis axially displaceable relative to the driver element and includesteeth which engage coarse threads provided in the driven elements. Theteeth are self locking such that rotation of the tube causes the drivenelements to rotate. Translational movement of the tube results insuperimposed rotation of the driven element due to the engagement of theteeth in the threads. A fluid linkage is provided to cause translationalmovement of the tube in response to predetermined values.

From the foregoing copendium, it will be appreciated that it is aconsideration of the present invention to provide a tacked vehicle drivesystem of the general character described which is not, however, subjectto the disadvantages of the backround art aforementioned.

It is a further consideration of the present invention to provide atracked vehicle drive system of the general character described which iscapable of furnishing high speed travel with good steering control andcontinuously variable speed propulsion at low vehicle speeds.

It is a feature of the present invention to provide a tracked vehicledrive system of the general character described which is capable ofvehicle propulsion at low speeds along a straight course withoutdeviations due to manufacturing tolerances of individual drive systemcomponents or ground conditions.

It is an aspect of the present invention to provide a tracked vehicledrive system of the general character described which is relatively lowin cost and capable of economical mass production fabrication.

A further aspect of the present invention is to provide a trackedvehicle drive system of the general charcter described which is highlyversatile and relatively simple in construction.

A further consideration of the present invention is to provide a trackedvehicle drive system of the general character described which utilizesdifferent drives for high speed and low speed propulsion while utilizingthe same steering linkage for steering control with either drive.

Another aspect of the present invention is to provide a tracked vehicledrive system of the general character described having a control elementsetting apparatus for operation of a control element as a function ofboth an operator steering control and predetermined vehiclerequirements.

Another consideration of the present invention is to provide a trackedvehicle drive system of the general character described which assuresthat each of the vehicle's tracks are traveling at exactly the samespeed.

An additional consideration of the present invention is to provide atracked vehicle drive system of the general character described whichincludes a steering drive capable of providing forward vehiclepropulsion.

A further aspect of the present invention is to provide a controlelement setting apparatus of the general character described whichreceives at least two desired setting signals and mechanically transmitsto the control element a setting position which constitutes thedifferential thereof.

Other aspects, features and considerations of the invention in part willbe obvious and part will be pointed out hereinafter.

With these ends in view, the invention finds embodiment in certaincombinations of elements, arrangements of parts and series of steps bywhich the said aspects, features, and considerations and certain otheraspects, features and considerations are hereinafter attained, all asfully described with reference to the accompanying drawings and thescope of which will be more particularly pointed out and indicated inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings in which are shown some of the variouspossible exemplary embodiments of the invention;

FIG. 1 is a schematized diagrammatic representation of a tracked vehicledrive system constructed in accordance with and embodying the inventionand showing a propulsion drive engaging planetary gears and a steeringdrive which includes two hydrostatic units and a neutral shaft having aseparable coupling;

FIG. 2 is a schematized diagrammatic representation of the trackedvehicle drive system with the neutral shaft coupling closed foroperating the vehicle in a high speed "travel" mode;

FIG. 3 is a schematized diagrammatic representation of the drive systemwith the neutral shaft coupling opened for a low speed "work" mode;

FIG. 4 is a schematized diagrammatic representation of a modifiedembodiment of the invention wherein a differential reversing gear havinga pair of couplings is utilized to interconnect the neutral shaftsegments to provide the capability of a "special work" mode of vehiclepropulsion wherein the vehicle travels in a stabilized straight path;

FIG. 5 is a schematized block diagram illustrating control componentsdrive system; and

FIG. 6 is a fragmentary longitudinal sectional view through a controlelement setting apparatus for adjusting the position of the hydrostaticunit control elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a tracked vehicle, the track chains are operated at the same speedwhen driving straight forward. When traversing a curve, however, theyare driven at different speeds consistant with the radius of the curve.A drive system includes a propulsion drive including a speed changegearing, preferably shiftable under load for drive force (torque)transmission from an engine to chain wheels which engage track chains.The drive system also includes a steering drive having at least onehydrostatic unit with at least a fluid pump, driven by the engine and afluid motor driven by the pump fluid. The output of the fluid motor canbe varied by setting the delivery volume of the fluid pump. In additionwith variable fluid motors, the motor itself may have control settings.

The fluid motor output engages a neutral shaft which does not rotatewhen the vehicle is traveling straight forward. Depending on whether thevehicle is traversing a right or left curve, the neutral shaft is drivenby the fluid motor in one or the other direction. The neutral shaftrotation is superimposed on the rotation of the chain wheels through asumming planetary differential gear which interconnects the speed changegearing and each chain wheel. The chain wheels thus rotate at a speedwhich is a function of both the propulsion drive and steering driveoutputs.

The neutral shaft is separable into segments for particular operatingmodes of the drive system. By separating the neutral shaft it ispossible to drive the chain wheels solely through the neutral shaftsegments with both segments having the same speed for straight traveland different speeds for traversing a curve. Under such conditions, thespeed change gearing is rendered inoperative.

Referring now in detail to the drawings wherein like numerals denotelike embodiments, the reference numeral 5 denotes generally a trackedvehicle drive system constructed in accordance with and embodying theinvention. The drive system 5 is driven by a vehicle engine 10 which iscoupled to the drive system through a coupling 12, e.g. a clutch. Thecoupling 12 is connected to an intermediate transmission element 14. Theelement 14 engages a propulsion drive shaft 20 through a hydrodynamictorque converter 16, or, alternately, the drive shaft 20 is driventhrough a bridge coupling 18.

The drive shaft 20 constitutes an input shaft of a propulsion drive. Thepropulsion drive includes a speed change gearing 22 which is shiftableunder load. The speed chage gearing is driven through a beveled gear 26fixed to an end 24 of the drive shaft 20. The speed chage gearing 22includes a plurality of planetary gear sets having an output 28 at aplanet carrier 30. The carrier 30 is nonrotatably connected to a driveshaft 32.

One end, 38, of the drive shaft 32 is connected to a cage 42 of aplanetary gear (summing differential gear) 46. The other end, 40 of thedrive shaft 32 is connected to a cage 44 of a summing differential gear48. The drive shaft 32 is also connected to a secondary pump 34 and animpulse transmitter 36. In addition, the drive shaft 32 is connected toa hydrodynamic retarder brake 50 for the purpose of braking the vehicle.

A sun gear 52, 54 of the summing differential gear 46, 48 respectivelyis connected to a neutral shaft 60 of a steering drive through a geartrain 62, 64 respectively. The gear train 62 contains one gear more thanthe gear train 64 so that the sun gears 52, 54 rotate in oppositedirections.

A planet carrier 66, 68 of the summing differential gears 46, 48respectively constitutes the output element of each differential gear.The carrier 66 is fixed to a shaft 70 to which a chain wheel 78 issecured. Similarly, the carrier 68 is fixed to a shaft 72 to which achain wheel 80 is secured. On each of the shafts 70, 72, a mechanicalvehicle brake 74, 76 respectively is positioned for vehicle braking.

It should be appreciated that with respect to each differential gear 46,48 the cages 42, 44, planet carriers 66, 68 and sun gears 52, 54 may besubstituted for one another with regard to their functions as drivingelement, driven element and reaction element.

In accordance with the invention, the neutral shaft 60 is separable intotwo segments 82, 84 which are connected to each other by a switchablecoupling 86. With the coupling 86 closed, the shaft 60 functions as asingle shaft. In order to ascertain the rate and direction of rotationof the shaft segments 82, 84, a rotation pickup 92 is associated witheach segment and is interconnected to a control device 93.

Pursuant to the invention, the steering drive includes a pair ofcontinuously variable hydrostatic units, 94, 96. The hydrostatic units,94, 96 are driven through the intermediate driver 14 and a bevel geararrangement having a pair of output shafts 102, 104. The shaft 102 isconnected to an input of the hydrostatic unit 94 through an internal rimgear and the shaft 104 is connected to an input shaft 100 of thehydrostatic unit 96 through an internal rim gear.

Each of the hydrostatic units 94, 96 includes a variable fluid pump 95which drives an fluid motor 97. Preferably, the outputs of the fluidmotors 97 are variable.

An output 110 of the unit 94 is connected through a spur gear 114 withthe neutral shaft segment 82 and an output 112 of the unit 95 isconnected through a spur gear 116 to the neutral shaft segment 84.

The intermediate element 14 serves as a torque divider, distributinginput power from the engine 10, through the coupling 12, to thepropulsion drive shaft 20 and, to the inputs of the steering drivehydrostatic units 94, 96.

In addition, the driving torque from the intermediate element 14 istransmitted through a beveled gear (shown in dashed lines) to aventilator 111 via a hydrodynamic coupling 109. A plurality of oil pumps108, located on the primary side of the gear shift mechanism 22, aredriven through the drive shaft 20.

The speed change gearing 22 is preferably shiftable under load. As such,the gear shift mechanisms can be switched while being coupled to thevehicle engine. The beveled gear 26 permits changing the gear shiftmechanism from a forward drive, during which a switch coupling R isopened and a coupling V is closed, to a reverse drive by opening thecoupling V and closing the switch coupling R. The speed change gearing22 also has a switch coupling 1 for a first planetary gear, a switchcoupling 2 for a second planetary gear, and a switch coupling 3 for athird planetary gear.

By opening the switch couplings V and R the speed change gearing 22 isphysically separated from the propulsion drive shaft 20. By closing theswitch couplings 1 and 4 for the first and fourth gears, the drivenelements, that is, the cages 42, 44 of the differential gears 46, 48 arelocked. The fourth gear coupling 4 interconnects the sun gear and cageof the first stage planetary gear.

In FIG. 2 the tracked vehicle drive system 5 is illustrated in aconfiguration of operating in a travel mode for vehicle travel atrelatively high speeds. It will be seen that the neutral shaft coupling86 is closed and the segments 82, 84 of the shaft 60 rotate unitarily sothat the shaft 60 functions as a single piece shaft.

The driving torque available at the coupling 12 is separated at theintermediate element 14 into a propulsion drive torque and a steeringdrive torque. The flow of the propulsion drive torque is indicated byplurality of heavy arrows designated by the numeral 120. The flow of thesteering drive torque is illustrated by the dashed arrows designated122. The drive torque flow 120 proceeds from the intermediate element 14to the torque converter 16, to the speed change gearing 22 and from thespeed change gearing through the differential gears 46 and 48 to thechain wheels 78, 80 which drive the chain tracks.

For vehicle steering control, the steering drive torque flow 122proceeds from the intermediate element 14 to the hydrostatic units 94,96. The torque flow is transmitted by the fluid pumps 95 to the fluidmotors 97 and from the fluid motors, to the neutral shaft 60.

With the drive system 5 in the travel mode and the vehicle travelingalong a straight course, the functionally one piece neutral shaft 60does not rotate. This is accomplished by control of the fluid pumpdelivery volume and/or fluid motor setting device. When the vehicle istraversing a curved course, the neutral shaft is driven by thehydrostatic units 94, 96 in the same direction, with the direction ofrotation being dependant upon the direction of turn. From the neutralshaft 60, the steering torque flows through the gear trains 62, 64 whichdrive the sun gears 52, 54 in opposite directions. During the travelmode of operation, the major portion of the drive torque is utilized forvehicle propulsion and is transmitted through the speed change gearing22 and only a minor portion of the engine generated torque is utilizedfor steering. As a result, travel speed can be very high.

In FIG. 3 the drive system 2 is illustrated in an alternate operatingmode, i.e. a work mode, wherein the vehicle is traversing at relativelylow speeds. The neutral shaft coupling 86 is separated and the neutralshaft segments 82, 84 rotate independantly of each other. The steeringdrive torque flow 122 from the hydrostatic unit 94 is transmitted onlyto the neutral shaft segment 82 and the steering torque flow, 122 fromthe hydrostatic unit 96 is transmitted only to the neutral shaft segment84. In addition, in the work mode, the planetary gear cages 42, 44 ofthe gears 46, 48 are blocked by closing the switch couplings 1 and 4 ofthe first planetary set. This is symbolically illustrated in FIG. 3 bythe numeral 130 indicating an arrow from a frame component to the driveshaft 32. The drive shaft 32 can be blocked by utilizing a brake, key orother blocking components.

It should be additionally noted that the propulsion drive torque flow120 at the speed change gearing end 24 of the drive shaft 20 isinterrupted. Such interruption is accomplished by opening the switchcouplings V and R. In lieu of or in addition to opening the couplings Vand R, the propulsion torque flow 120 can be interrupted by emptyingfluid from the torque converter 16 and opening the bridge coupling 18.The end objective is to assure that no propulsion torque flow 120extends into the speed change gearing 22. Propulsion drive torque flowsfrom the intermediate element 14, through the hydrostatic units 94, 96to the respective neutral shaft segments, 82, 84. As a result, thehydrostatic units 94, 96 operate to propel the vehicle slowly during thework mode.

The driving torque and the steering torque can be divided through thehydrostatic units 94, 96 onto the chain wheels 78, 80 in a continuouslyvariable manner. Under extreme driving situations, it is possible totransmit the entire torque generated by the vehicle engine 10 to onlyone chain wheel 78, or 80. In such a manner, it is possible to "tearloose" a vehicle track or chain which is stuck.

The versatility of power transmission through the hydrostatic units 94,96 is such that the vehicle speed, in the work mode, can be reduced tozero or the specified optimum working speed of any auxiliary equipmentcarried on the vehicle. Such working equipment can include, minesweepers, ditch digging devices or other equipment. Further, the drivingspeed and direction of the vehicle can be adjusted as a function of theworking speed specifications of the auxiliary equipment by utilizing therotation pickups 92 for determining the instantaneous speed values.

In a further embodiment of the invention illustrated in FIG. 4, adifferential reversing gear 140 is employed in lieu of the coupling 86to interconnect the neutral shaft segments 82, 84. The differentialreversing gear includes a reversing gear 141 comprising a matrix ofintergaged bevel gears. In addition, the differential reversing gear 140includes a switchable first coupling 142 joining a casing with thesegment 84 and which is engagable to block the reversing gear 141 andprovide a unitary connection between the neutral shaft segments 82, 84.The gear 140 also includes a second switchable coupling 144.

When the second coupling 144 is closed, the shaft segments 82, 84 rotatein opposite directions with respect to each other with the neutralshaft, however, functioning as a single unit. With the second couplingopen, the differential reversing gear 140 will provide the travel andwork modes. When the first coupling 142 is closed, the vehicle drivesystem is placed in the travel mode with the propulsion drive torqueflow identical to that of FIG. 2. With the first coupling 142 opened,the vehicle drive system operates in the work mode and the torque forceflows 120 and 122 identical to those illustrated in FIG. 3 result.During the work mode, the hydrostatic unit 94, 96 only drives itsrespective neutral shaft segments 82, 84. The units 94, 96 drive thesegments 82, 84 during forward motion of the vehicle in rotationaldirections opposite to one another since a rotational direction reversalat the sun gears 52, 54 takes place due to the different number of gearsin the gear train 62, 64.

In addition, a third operating mode of the vehicle drive system, thespecial work mode, is available when the first coupling 142 is openedand the second coupling 144 is closed. In such configuration, theneutral shaft segments 82, 84 are functionally connected through thereversing gear 140. They rotate, in opposite rotational directions andtorque force flows substantially identical to those of the work mode andas illustrated in FIG. 3 are obtained. Significantly, however, the forceflows 120 and 122 reinforce each other through the differentialreversing gear 140 in such a way that the vehicle will only drivestraight forward.

In the special work mode, the forward motion of the vehicle isstabilized because nonuniform drive resistances on the chains of thetrack vehicle and the force flows of the hydrostatic units 94, 96 aredivided proportional to the chain wheel resistance through the reversinggear 141. As a result, both track chains drive with uniform travelspeed. In extreme cases, both hydrostatic units, 94, 96 act togetherupon only one track chain. Thus, straight forward driving in the specialwork mode is insured even if the vehicle is driving on a slope or withone of the track chains on a slippery surface such as an ice coveredsurface. The chain wheels 78, 80 are always assured of having the samerotation speed. The special work mode of the vehicle drive system alsoassures that minor errors in setting the hydrostatic untis 94, 96 aswell as errors in hydrostatic absorption capacity are automaticallycompensated.

FIG. 5 illustrates details of the control unit 93. Schematically shownin such illustration are the fluid pumps 95 and the fluid motors 97 ofthe hydrostatic units, 94, 96. Each fluid pump 95 includes a controlelement 202 which comprises a volume setting lever. The levers 202 areeach connected for adjustable rotational movement to an acuating lever208, 209 of a setting apparatus 230 by a link arm 206. In addition, eachof the fluid motors 97 has controls which are actuated by a settingdevice 212 for adjusting driven speed. The setting devices 212 arehydraulically actuated through a pair of fluid lines 214 from a controlapparatus 210.

The control apparatus 210 is supplied with oil from the speed changegearing 22 through a supply line 216 while a return line 218 serves fordischarge flow of oil back to the gearing 22. With the vehicle drivesystem operating in its travel mode as illustrated in FIG. 2, the speedchange gearing 22 is switched through an electronic gear control circuit222 which responds to the detected position of a speed selection switch220.

The operator set position of a steering wheel 224 is mechanicallytransmitted through a linkage 226 to lever 228 of a setting apparatus230. The setting apparatus 230, in a manner to be hereinafter described,mechanically transmits the steering control position of the steeringwheel 224 to the control elements 202 of the fluid pumps 95.

With the vehicle drive system 5 in the work mode, the neutral shaftsegments 82, 84 are functionally separated from each other and thepropulsion drive torque flow does not proceed through the speed changegearing 22. Both the propulsion drive torque and the steering drivetorque proceed through the hydrostatic units 94 and 96 to the neutralshaft segments 82, 84 and from such segments, to the differential gears46 and 48. For this reason, the driving speed can not be set through thespeed change gearing 22 but only through the hydrostatic units 94, 96.Thus, the control elements 202 of the fluid pumps 95 are now required tobe set not only as a function of the steering wheel position but also asa function of the desired slow driving speed. The setting apparatus 230serves to combine both the vehicle steering requirements of the wheelposition and the driving speed requirements into hydrostatic unitcontrol element positions.

A mode selection switch 232 is provided for operator selection of thetravel or work modes of the vehicle drive system. In the travel positionof the selection switch 232, the special work mode will be activatedwhen the steering wheel is moved to a particular forward position. Thisresults in automatically closing the second coupling 144 while the firstcoupling 142 remains open.

The mode selection switch 232 is coupled to a component of workingequipment 234, e.g. a mine detector or other auxiliary workingequipment, carried by the vehicle. During the work or special work modeof the drive system, a resulting signal indicative of the desiredvehicle speed for the equipment is transmitted along a line 236 to thegear control circuit 222. The gear control circuit 222 includes acomparator which compares the desired speed signal value with the actualspeed signal value (generated by the rotational pickups 92). A pair ofelectrical lines 238 and 239 interconnect the pickups 92 with thecontrol circuit 222. The control circuit 222 then transmits anelectrical speed setting signal to the control apparatus 210 through anelectrical line 242.

The control apparatus 210 receives the speed setting signals and, thesetting apparatus 230, converts such signals to mechanical settingvalues which are superimposed on the steering wheel setting values. As afunction of such superimposition the control elements 202 of thehydrostatic units 94, 96 are moved into positions corresponding to therequired speed of the vehicle as well as steering direction set at thesteering wheel 224.

The desired vehicle speed for work and special work modes can beoptionally set through a manual speed setting control 244. The workingequipment 234 also transmits signals to an engine control device 246,e.g. throttle control, for adjustment of the vehicle engine speed inaccordance with the requirements of the working equipment. The magnitudeof the desired speed signal transmitted to the gear control circuit 222along the line 236 constitutes a function of both the setting of themanual control 244 and the most appropriate constant engine speed. Thevehicle is provided with an instrument panel 248 which serves todisplay, among other things, the operational states of both the workingequipment 234 and the gear control circuit 222.

With the drive system in the work or special work operating modes, thehighest vehicle driving speed is attained when the control elements 202have reached their maximum settings. If at the highest vehicle speed, acurve is to be traversed, at least one of the two control elements 202must be reduced to a lower speed so that a speed difference resultsbetween the track chains of the vehicle.

A sensor 250 is associated with the steering wheel and produces signalsindicative of the steering wheel position. Such position signals arereceived at the gear control circuit 222 through an appropriateelectrical line. The control apparatus 210, through the settingapparatus 230, serves to decrease the speed of one of the chain tracksonly to the extent required for traversing the curve as set at thesteering wheel. The position of the control element 202 associated withthe hydrostatic unit 94 and the control element 202 associated with thehydrostatic unit 96 is sensed by an angle sensor 251, 253 respectively.The signals generated by such sensors are received at the gear controlcircuit 222 and are compared, in the gear control circuit, with thesignal of the rotation pickups 92 in order to avoid hunting, i.e.overcompensation and adjustment fluctuations.

Further details of the control apparatus 210 and the setting apparatus230 are illustrated in FIG. 6. The control apparatus 210 includes thesetting apparatus 230 and a proportional hydraulic valve 262 which isinterconnected to the setting apparatus 230 through a switching valve260. The proportional valve 262 comprises a valve known as a MOOG typevalve and generates a pressure differential in a pair of hydraulicoutput lines 264, 265. The pressure differentials in the lines 264, 265are generated as a function of the electrical signals appearing on theline 242 which extends from the gear control circuit 222. In actualitythe line 242 constitutes a plurality discrete electrical signal lines.The pressure differential on the lines 264, 265 effects and maintainsdesired setting in the setting apparatus 230 which corresponds to thedesired driving speed of the vehicle.

The setting apparatus 230 includes an elongate casing 266 within which agenerally cylindrical driver 268 is coaxially carried. The driver 268 isrotatably mounted to the casing 266 through an upper bearing 270. Fixedto the upper end of the driver 268 is the steering wheel acuated controllever 228. Rotation of the steering wheel 224 results in correspondingrotation of the driver 268.

The surface of the driver 268 includes longitudinal keyways 272 and thedriver projects into a coaxial connecting tube 274. Mounted to andprojecting radially inward from the tube 274 are a plurality of keys 275which are engaged in the keyways 272. As a result, the connecting tube274 rotates with but is axially slidable relative to the driver 268.

Concentrically positioned within the driver 268 is a hollow drivenelement 290 and concentrically positioned within the driven element 290is a further driven element 288. The actuating lever 208 is securedadjacent an end of the driven element 288 and the actuating lever 209 issecured adjacent an end of the driven element 290. At the opposite endof the driven element 290 there is an enlarged cylindrical zone 276having a threaded section 280. Similarly, the driven element 288includes an enlarged cylindrical zone 282 having a threaded section 278.

The threaded section 280 includes coarse threads 281 formed on itssurface and the threaded section 278 includes coarse threads 283 on itssurface. A pair of carrier teeth 284 projecting from the tube 274 engagethe coarse threads 281 and a pair of carrier teeth 286 projecting fromthe tube 274 engage the coarse threads 283. When the connecting tube ismoved axially relative to the driver 268, the engagement between thecoarse threads 281, 283 and their respective carrier teeth 284, 286result in rotation of the threaded sections, 280, 278, hence rotation ofthe driver elements 290 and 288 relative to the connecting tube 274.

The coarse threads 281, 283 are inclined in different rotationaldirections, i.e. one being a right handed thread and the other being aleft handed thread, so that the threaded sections 280, 278 rotate inopposite directions. As previously mentioned, each of the threadedsections 280, 278 is part of a driven element 288, 290 respectively. Thedriven elements include the actuating levers 208, 209. As shown in FIG.6, the driven element 288 constitutes a rod which extends through thecylindrical zone 276 and threaded section 280 of the driven element 290.

It should be appreciated that variations of the setting apparatus areinvisioned wherein fewer or more than two threaded sections 276, 280will be available. In addition, the coarse threads 281, 283 can extendin the same rotational direction depending upon the desired direction ofmovement of the actuating levers 208 and 209.

It should thus be appreciated that the connecting tube 274 serves torotatably connect the driver element 268 with the driven elements 288,290 through the threaded sections 278, 280. As a result, the driverelement 268, the threaded sections 280, 278 and the connecting tube 274are rotated together when the driver element 268 is rotated throughsteering wheel movement at the lever 228. Such rotation motion of thedriver element 268 can be superimposed on the axial position of theconnecting tube 274 in the work and special work modes of the vehicledrive system. The desired driving speed corresponds to a givenconnecting tube axial position in the work and special work modes. Anaxial displacement of the connecting tube 274 results in a correspondingrotation of the threaded sections 278, 280 due to the engagement betweenthe carrier teeth 284, 286 and the threaded sections 278, 280. Suchrotation of the threaded sections is in opposite directions of rotationwith respect to each other and does not change the set position eitherangularly or axially of the driver 268. Because of the driver 268 is notaffected, the rotational speed difference at the outputs 110 and 112 ofthe hydrostatic units 94 and 96 which is set for a particular curve willnot be affected.

It should be noted that the coarse threads 281, 283 are self locking inthe direction of rotation. Rotation of the connecting tube 274 due torotation of the driver 268 does not result in rotation of the threadedsections 278, 280 relative to the connecting tube, but the two sections278, 280 rotate together with the connecting tube 274, thus maintainingtheir angular positions relative to the connecting tube. Only axialdisplacement of the connecting tube 274 causes rotation of the threadedsections 278, 280. The keys 275 and the carrier teeth 284, 286 arepreferably formed of two parts, rotatable relative to each other in adirection transverse to the longitudinal axis of the setting apparatus230. As a result freeplay in the direction of rotation is eliminated.

The driven element 288 is rotatably mounted within the driven element290 by a pair of bearings 291. Similarly the driven element 290 isrotatably mounted within the driver 268 by a pair of bearings 292.

In order to provide axial displacement of the connecting tube 274relative to the driver element 268 and the threaded sections 280, 278various systems may be utilized, including mechanical, hydraulic,pneumatic or electrical or any combination thereof. In the typicalconfiguration illustrated in FIG. 6, the axial displacement of theconnecting tube is hydraulically actuated as a function of electricalsignals transmitted from the gear control circuit 222.

Beneath the connecting tube 274, a cylinder 300 is positioned. Adjacentthe upper end of the cylinder 300, a lower zone 296 of the connectingtube 274 is rotatably coupled to the cylinder through a bearing 298. Thebearing 298 permits only rotational movement between the cylinder andthe connecting tube so that axial displacement of the cylinder 300results in corresponding axial displacement of the connecting tube. Apiston 302 is positioned in the cylinder 300 and the piston is fixedrelative to a stationary base 304 of the outer casing 266 by a pistonrod 306 which extends to an aperture in the cylinder bottom wall 308.The cylinder, piston and piston rod are coaxial with the connecting tube274. The piston 302 divides the cylinder into an upper pressure chamber312 and a lower pressure chamber 310.

A hydraulic line 264/2 constitutes a continuation of the line 264between the switch valve 260 and the pressure chamber 310. Similarly, aline 265/2 extends as a continuation of the line 265 from the switchvalve 260 to the pressure chamber 312.

The switch state of the valve 260 illustrated in FIG. 6 is that of thetravel mode of the vehicle drive system. As such, the lines 264 and 265are interrupted and the lines 264/2 and 265/2 are connected to a drainor sump 316. As a result, the pressure chambers 310, 312 are empty and apair of springs 320, 322 maintain the cylinder, hence the connectingtube 274, in a central position. Rotation of the control element 228 istransmitted directly to the actuating levers 208 and 209 and asuperimposition of other motions i.e. desired speed, does not takeplace.

For the control apparatus 210 to superimpose on the position of thelever 228 an additional value affecting the output of the hydrostaticunits 94, 96, an electrical velocity signal is transmitted to theproportional valve 262 and a signal is transmitted to the switch valve260. This occurs during the work and special work modes. When the switchvalve 260 is so actuated, the hydraulic lines 264, 265 from theproportional valve 262 are directly connected to their associated lines264/2 and 265/2 to introduce pressure fluid into the chambers 310, 312.The cylinder 310 is moved axially relative to the stationary piston 302.Such displacement of the cylinder 310 corresponds to the electricalsignal transmitted to the proportional valve 262.

The connecting tube 274 moves in unison with the cylinder 300 and causesthe threaded sections 276, 280 to rotate, hence the actuating levers208, 209 will rotate. The rotational position of the levers 208, 209correspond to a position in which the hydrostatic unit control elements202 generate outputs corresponding to the desired driving speed.

When an operator rotates the steering wheel 224, the link 226 causes thelever 228 to correspondingly rotate. The rotation speed at the outputs110, 112 of the hydrostatic units 94, 96 will then deviate from thevelocity set by the axial displacement of the tube 274 in order totraverse the curve over which the vehicle is traveling.

A connecting tube sensor 330 is mounted to the casing 266 of the settingapparatus 230 and generates electrical signals as a function of the tubeaxial position, i.e. the set driving speed of the vehicle. The sensormay comprise any of various available position sensing devices such asan optical detector. The gear control circuit 222 receives the signal ofthe sensor 330 and determines whether a hydrostatic units 94, 96 areoperating at their maximum output speed.

With the drive system in the work mode, propulsion is provided onlythrough the hydrostatic units 94, 96. To turn the vehicle in response toa steering wheel setting, the hydrostatic unit which drives the chain onthe outside arc of the curve must rotate at a faster speed than thehydrostatic unit which drives the chain on the inside arc of the curve.If a vehicle turn is indicated by the steering wheel setting when thesignal received from the sensor 330 indicates that the hydrostatic unitsare at a maximum speed, the speed of the hydrostatic unit which drivesthe chain on the inside arc of the curve is reduced to effect the turn.

A source 334 of pressurized hydraulic fluid which may comprisepressurized fluid supplied through the secondary pump 34 or the pumps108 of the speed change gear 22, supplies the proportional valve 262with hydraulic fluid through a fluid line 216. An output 340 of apressure reducing valve 336 is connected to the fluid lines 214 of thefluid motor setting devices 212 through a fluid line 342. The fluidlines 214 each supply fluid to a setting cylinder 344, 346 of one of thesetting devices 212 for adjusting the driven speed of the respectivefluid motor 97. In the lower and medium speed range of the vehicle, thehydraulic pressure in the cylinder 344 and 346, hence the setting of thefluid motors 97, is maintained at a constant value. With the vehicledrive system operating in the work or special work modes, in the upperspeed range, a radial projection 346 adjacent the top of the cylinder300 contacts an abutment actuator 348 of the proportional valve 336.Further downward displacement of the cylinder 300 causes the abutmentactuator 348 to acutate the pressure reducing valve 336 and effect achange in the pressure of the fluid supply to the setting cylinders 344and 346. As a result, the driving velocity of the vehicle is no longerdetermined by the setting of the fluid pumps 95 alone but additionallyas a result of the adjustment setting of the fluid motors 97.

It should be appreciated that readily apparent modifications of thesetting apparatus 230 can include replacing the cylinder 300 and piston302 with an electric, hydraulic or pneumatic motor or displacementdevice or any other readily available setting elements. A significantadvantage is that the electrical signal values generated by the gearcontrol circuit 222 are converted to mechanical values and, asmechanical values are superimposed on the mechanical value of thesteering setting lever 228. A further advantage of the present inventionis that the angular adjustment of the lever 228 results in thegeneration of more than a single output motion which can be of differentdirections depending upon the number of threaded sections 278, 280employed and can have different directions of movement, depending uponthe pitch and thread direction of the coarse threads 281, 283.

Further, the control apparatus 210 and the setting apparatus 230 are notconfined to applications in setting hydrostatic units but can be appliedfor the setting of many other devices which are required to be set as afuction of at least two control values. Further, in accordance with thepresent invention, the setting apparatus 230 provides for purelymechanical transmission of the setting motions of the control lever 228thus assuring a high degree of functional safety as compared withelectrical, pneumatic or hydraulic apparatus.

Thus, it will be seen that there is provided a tracked vehicle drivesystem which achieves the various aspects, features and considerationsof the present invention at which is well suited to meet the conditionsof practical usage.

Since various possible embodiments might be made of the presentinvention and various changes might be made in the exemplory embodimentsset forth, it is to be understood that all material shown and describedin the accompanying drawings is to be interpreted as illustrative andnot in a limiting sense.

Having thus described the invention there is claimed as new and desiredto be secured by Letters Patent:
 1. A tracked vehicle drive system,including:(a) a propulsion drive having a speed change gearing with anoutput, (b) a steering drive having hydrostatic torque transmittingmeans, the steering drive having an output, (c) means drivinglyinterconnecting an engine output with the propulsion drive and thesteering drive, (d) a pair of summing differential gears, the gears eachhaving a first input element and a second input element, means drivinglyinterconnecting the speed change gearing output with the first inputelement of each summing differential gear, means drivinglyinterconnecting the second input element of each summing differentialgear with the steering drive output, each summing differential gearfurther having an output element, each output element rotating at aspeed which is a function of the speeds of the two input elements, and(e) a neutral shaft and means connecting the second input element ofeach summing differential gear with the neutral shaft,characterized inthat: (f) the neutral shaft is comprised of segments, the drive systemincluding switchable means for selectively interconnecting the neutralshaft segments, the segments being connected when the vehicle drivesystem is in a travel mode and being uncoupled when the drive system isin a work mode adapted for slow speed travel, (g) the speed changegearing is comprised of a multiple stage shiftable gearing and the meansdrivingly interconnecting the speed change gearing output with the firstinput element of each summing differential gear comprises a unitarypropulsion drive shaft, (h) the hydrostatic torque transmitting meanscomprising two continuously variable hydrostatic units, the meansdrivingly interconnecting the engine output with the steering driveincluding means operatively engaging an input of each hydrostatic unit,the second input element of one summing differential gearing beingnonreleasably drivingly engaged with the output of one hydrostatic unitand the second input element of the other summing differential gearbeing nonreleasably drivingly engaged with the output of anotherhydrostatic unit and, (i) the system further including means forselectively disengaging the speed change gearing from the meansdrivingly interconnecting the propulsion drive with the engine outputand means for selectively blocking the first input element of thesumming differential gears from rotating, the speed change gearing inputbeing interrupted and the first input elements being blocked when thedrive system is in a work mode, the steering drive transmitting, throughthe hydrostatic units, the propulsion drive torque for advancing thevehicle when in the work mode.
 2. A tracked vehicle drive systemconstructed in accordance with claim 1 wherein the speed change gearingincludes a forward and a reverse switchable coupling and switchable gearstages, the speed change gearing being selectively disengaged by openingthe forward and reverse couplings and the first input elements beingselectively blocked by simultaneously engaging two gear stages.
 3. Atracked vehicle drive system, including:(a) a propulsion drive having aspeed change gearing with an output, (b) a steering drive havinghydrostatic torque transmitting means, the steering drive having anoutput, (c) means drivingly interconnecting an engine output with thepropulsion drive and the steering drive, (d) a pair of summingdifferential gears, the gears each having a first input element and asecond input element, means drivingly interconnecting the speed changegearing output with the first input element of each summing differentialgear, means drivingly interconnecting the second input element of eachsumming differential gear with the steering drive output, each summingdifferential gear further having an output element, each output elementrotating at a speed which is a function of the speeds of the two inputelements, and (e) a neutral shaft and means connecting the second inputelement of each summing differential gear with the neutralshaft,characterized in that: (f) the neutral shaft is comprised ofsegments, the drive system including switchable means for selectivelyinterconnecting the neutral shaft segments, the segments being connectedwhen the vehicle drive system is in a travel mode and being uncoupledwhen the drive system is in a work mode adapted for slow speed travel,(g) the hydrostatic torque transmitting means comprising twocontinuously variable hydrostatic units, the means drivinglyinterconnecting the engine output with the steering drive includingmeans operatively engaging an input of each hydrostatic unit, the secondinput element of each summing differential gear being drivable by theoutput of a hydrostatic unit and (b) the system further including meansfor selectively disengaging the speed change gearing from the meansdrivingly interconnecting the propulsion drive with the engine outputand means for selectively blocking the first input element of thesumming differential gears from rotating the speed change gearing inputbeing interrupted and the first input elements being blocked when thedrive system is in a work mode, the steering drive transmitting, throughthe hydrostatic units, the propulsion drive torque for advancing thevehicle when in the word mode, (i) the means for selectivelyinterconnecting the neutral shaft segments comprising a differentialreversing gear having a reverse gear, a first coupling and a secondcoupling whereby when both couplings are open the neutral shaft segmentsare functionally separted for the work mode, when the first coupling isclosed and the second coupling opened the neutral shaft segments arefunctionally connected for the travel mode and when the first couplingis opened and the second coupling is closed, the neutral shaft segmentsare functionally connected through the reversing gear for a stabilizedword mode wherein the driving force is automatically apportioned overthe neutral shaft segments for compensation of different resistances. 4.A tracked vehicle drive system, including:(a) a propulsion drive havinga speed change gearing with an output, (b) a steering drive havinghydrostatic torque transmitting means, the steering drive having anoutput, (c) means drivingly interconnecting an engine output with thepropulsion drive and the steering drive, (d) a pair of summingdifferential gears, the gears each having a first input element and asecond input element, means drivingly interconnecting the speed changegearing output with the first input element of each summing differentialgear, means drivingly interconnecting the second input element of eachsumming differential gear with the steering drive output, each summingdifferential gear further having an output element, each output elementrotating at a speed which is a function of the speeds of the two inputelements, and (e) a neutral shaft and means connecting the second inputelement of each summing differential gear with the neutralshaft,characterized in that: (f) the neutral shaft is comprised ofsegments, the drive system including switchable means for selectivelyinterconnecting the neutral shaft segments, the segments being connectedwhen the vehicle drive system is in a travel mode and being uncoupledwhen the drive system is in a work mode adapted for slow speed travel,(g) the hydrostatic torque transmitting means comprising twocontinuously variable hydrostatic units, the means drivinglyinterconnecting the engine output with the steering drive includingmeans operatively engaging an input of each hydrostatic unit, the secondinput element of each summing differential gear being drivable by theoutput of a hydrostatic unit, (h) the system further including means forselectively disengaging the speed change gearing from the meansdrivingly interconnecting the propulsion drive with the engine outputand means for selectively blocking the first input element of thesumming differential gears from rotating, the speed change gearing inputbeing interrupted and the first input elements being blocked when thedrive system is in a work mode, the steering drive transmitting, throughthe hydrostatic units, the propulsion drive torque for advancing thevehicle when in the work mode and (i) setting means for regulation ofthe vehicle driving speed and steering control, the setting meansincluding:(i) driver means and means mounting the driver means forrotation in response to the steering control, (ii) a driven element andmeans mounting the driven element for rotation coaxial with the drivermeans, means forming at least one coarse thread in the driven element,and means mechanically interconnecting the driven element with a controlof a hydrostatic unit, (iii) a connecting tube, means mounting theconnecting tube for rotation coaxial with the driver means and axiallydisplaceable relative to the driver means, the tube having carrier meansfor engaging the coarse thread, the carrier means including means foreffecting rotation of the driven element in response to axialdisplacement of the tube and for locking to prevent rotation of thedriven element from axially displacing the tube, and (iv) means foraxially displacing the tube in response to electrical signalscorresponding to a travel speed to be set.